One prior art solution is to use a voltage transformer to reduce the voltage to be measured to a SELV level and isolate the circuits from each other, as well. For current measurements, the signal level is converted and isolation carried out by using a current transformer, correspondingly. The large size and high price of the transformer are disadvantages. In applications susceptible to vibration, the heavy transformer and its electric connections may become detached from the printed circuit board.
When an electronic circuit is at a floating potential or is isolated from the main circuit using a high-impedance coupling, signalling and connections to the SELV circuit can be implemented using components with the required dielectric strength. In practice, solutions employing resistive voltage division have been common in three-phase networks having a voltage of no more than 400 V, as the power loss in the voltage division resistors and the dielectric strength required of the isolation transformers have led to large-size solutions.
In connection with the design of power distribution system equipment for the “Overvoltage Category III” or higher, and environments with “Pollution Degree III”, the requirements for creepage and clearance distances mean that signal and power supply components suitable for the SELV interface of conventional floating solutions are expensive or not available at all.
A floating electronic measurement circuit can also be earthed through a high-impedance coupling, but care must be taken to ensure that the number of devices connected in parallel does not become too high to bring the earth resistance of the electrical system below the limit specified in the regulations.